Heat pipe with dual working fluids

ABSTRACT

In a heat pipe containing a main working fluid that normally freezes under low heat loads, an auxiliary working fluid is provided that, although being less efficient than the main working fluid, nevertheless remains liquid at low heat loads when the main working fluid freezes, so as to sustain heat pipe action.

United States Patent 1 Shcosinger Dec. 11, 1973 HEAT PIPE WITH DUALWORKING FLUIDS [75] Inventor: Arnold P. Shcosinger, Los Angeles,

Calif.

[73] Assignee: TRW lne., Redondo Beach, Calif.

[22] Filed: June 1, 1970 [21] Appl. No.: 42,088

[52] US. Cl. 165/105, 165/134 [51] Int. Cl. F28d 15/00 [58] Field ofSearch 165/105, 134, 107

[56] References Cited UNITED STATES PATENTS 2,372,502 3/1945 Lehane eta1. 165/154 X 3,554,183 1/197l- Grover et a1 165/105 X 11/1971 Busse165/105 X 3,535,562 10/1970 Byrd 310/4 3,429,122 2/1969 Pravda et a1.60/3951 R 3,450,195 6/1969 Schnacke 165/105 X 3,532,158 [0/1970l-liebert 165/105 X Primary Examiner-Albert W. Davis, Jr.Attorney-Daniel T. Anderson, Jerry A. Dinardo and Donald R. Nyhagen [57]ABSTRACT In a heat pipe containing a main working fluid that normallyfreezes under low heat loads, an auxiliary working fluid is providedthat, although being less efficient than the main working fluid,nevertheless remains liquid at low heat loads when the main workingfluid freezes, so as to sustain heat pipe action.

10 Claims, 4 Drawing Figures Pmmin w 1 W5 A rnold P Shlosinger INVENTOR.

AGENT HEAT PIPE WITH 'DUAL WORKING FLUIDS BACKGROUND OF THE INVENTION 1.Field of the Invention This invention relates to heat pipes generallyand more particularly to heat =.pipes thatwill continue to functionunder low heat load conditions.

2. Description of the Prior Art Heat pipes or heat pipe-type devices can.be defined as devices employing closed evaporating-condensing cyclesfor transporting heat from a locale of heat generation to a locale ofheatrejection and using acapillary structure or wick for return of thecondensate. Such devices consist of a closed container which maybe ofany shape or geometry. They often have the shape of apipe or tube,closed on both ends, and the term heat pipe was derived from suchdevices. The term heat pipe is, however, in the present specificationand claims used in a more general sense to refer to devices of any typeof geometry that are designed to function as described.

In such adevice, air or other noncondensible gases are removed from theinternal cavity of the container.

' All interior surfaces are lined with a capillary structure,

suchas a wick. The wick is soaked with a fluid which will be in theliquid phase at the normal working temperature of the device. The freespace of the cavity then contains only the vapor of the fluid, at apressure corresponding to the saturation pressure of the working fluidat'the temperature of the device. If, at any location, heat is added tothe container, the resulting temperature rise will increase the vaporpressure of the working fluid and evaporation of liquid will take place.The vapor formed, being at a higher pressure, will flow towards thecolder regions of the container cavity and will condense on the coolersurfaces of the wick on the inside of the container wall. Capillaryeffects will cause the liquid condensate to return to the areas of heataddition. Because the heat of evaporation is absorbed by the phasechange from liquid to vapor and released when condensation of the vaportakes place, large amounts of heat can be transported, with very smalltemperature gradients, from areas of heat addition to areas of heatremoval. The foregoing is well-known and heat pipes have been recognizedfor several years as very effective heat transport devices. They willtransport large amounts of heat with small temperature gradientsindependent of gravity effects, which makes these devices suitable forapplications in space.

The requirements for desirable working fluids for heat pipe devicesinclude properties such as high surface tension which will enhancecapillary pumping, high heat of vaporization, and a freezing point ofthe working fluid above the lowest temperature which may occur at thecold areas of the device. It is this last requirement that has severelylimited the application of heat pipe-type devices to heat transport inspace. Unfortunately, the known more efficient working fluids includethe liquid metals and water, which have high freezing points, whereasthe lower freezing point fluids such as ammonia, the alcohols, theFreons and cryogenic fluids are less effective and less desirable.Solidification of the working fluid at the cold areas of a heat pipestops operation of the heat pipe. When freezing occurs in a heat pipe,the vapor pressure in the cold area drops. This results in evaporationof the working fluid in the warmer areas and flow of the vapor to thecold areas. As a result of this process, all the working fluid moves tothe areas of the heat pipe which are below the'solidification point ofthe working fluid, and the wick in the warmer areas will be completelydeprived of liquid and dry out. Increase of heat flow into the warmareas of the device does not then result in effective transfer of heatto the colder areas. No working fluid in the liquid state is availablefor evaporation and transport of heat by vapor flow to the colder areas.Theoretical and experimental investigations have shown that once a heatpipe is frozen at its cold areas, only conductive heat transfer in thecontainer wall is available to restart the heat pipe. Very largetemperature differences between the warm and cold areas of frozen heatpipes have been observed.

SUMMARY OF THE INVENTION The principal object of the invention is toovercome the foregoing drawbacks. In accordance with the invention, anefficient working fluid is used as the main heat pipe working fluid athigh heat transfer loads. When heat transfer loads diminish to the pointwhere the fluid in the colder areas of the heat pipe solidifies, a lowfreezing point auxiliary working fluid is available to transfer heat tothe cold areas of the heat pipe. Because the large heat flow rates aretaken care of by an efficient, although high freezing point workingfluid, a low freezing point auxiliary fluid can be used, even if itsheat transport capability is relatively poor. Heat flow at a low ratewill be maintained. When heat transfer loads increase, the auxiliaryworking fluid provides enough heat flow to initiate melting of thesolidified main working fluid. The main working fluid, after it has beenliquified, then returns by capillary action to the warmer areas of theheat pipe where it evaporates and transports additional amounts of heatto the cold areas, further helping in the melting of the working fluidin the cold areas. Thereby, a heat pipe in which the main working fluidhas become solid during a period of low heat load is able to start againwhen heat loads increase.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a longitudinal sectional viewof a dual section heat pipe employing two different freezing pointworking fluids according to the invention;

FIG. 2 is a section taken along line 2-2 of FIG. 1;

FIG. 3 is a side elevational view partly in section of another form ofheat pipe employing a two part wick structure and two different freezingpoint working fluids according to the invention; and

FIG. 4 is a section taken along line 4-4 of FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIGS. 1 and 2,there is illustrated a dual section heat pipe 10, there being a main,larger, outer section 12 enclosing an auxiliary, smaller, inner section14. The outer section 12 has a larger crosssection than the innersection 14, but both sections 12 and 14 have the same length. Althoughthe two sections 12 and 14 of the heat pipe 10 are illustrated as eachhaving a circular cross-section, they may be rectangular, square, or ofother operable geometry. The heat v pipe 10 is hermetically sealed atits ends, as by plates 16, so that the interior of the outer section 12is isolated from the interior of the inner section 14.

The interior surface of the outer section 12 is lined with capillarymaterial, or a wick 18. Quartz fiber, metal screens, or other woven orsintered materials may be used for the wick 18. The wick 18 may bejoined to the surface by means including bonding, soldering, spotwelding, or spring tension of the wick material.

The outer surface of the inner section 14 is over its entire length inthermal and physical contact with the wick l8 and is preferably joinedthereto. The inner section 14 may likewise be made of copper and may bejoined to the wick 18 by means of longitudinally spaced spring wiresupports 19 or similar means. Similarly, the interior surface of theinner section is lined with a wick 20, such as quartz fiber, metalscreens, or other woven or sintered material joined thereto by means ofbonding, soldering, spot welding or spring tension of the wick material.

It is preferable that a substantial outer surface area of the innersection 14 be in contact with the wick 18 of the outer section 12 so asto provide good thermal coupling therebetween. Thus, where the outersection 12 is circular in cross-section, the inner section 14 haspreferably a flattened or oval cross-section to afford contact along awide circular arc of the wick 18. However, other geometricalconfigurations may be used, provided there is sufficient area of thermalcontact between the two sections 12 and 14.

Both sections 12 and 14 are evacuated of all noncondensible gases. Bothwicks l8 and 20 are saturated with working fluid. However, two differentworking fluids are used. The working fluid used in the outer section 12is chosen for its high efficiency as a thermal transport medium and willbe referred to as the main working fluid. Usually, such a working fluidhas too high a freezing point to operate properly under the lowtemperature and low heat load requirements that are met in outer space,for example. Water and some of the liquid metals, such as potassium andlithium, have desirable heat transport qualities that make them suitablefor the working fluid in the outer section 12, which serves as the mainheat pipe operating section under normal temperature and heat loadconditions.

The working fluid in' the inner section 14 is selected principally forits low freezing point and secondarily for its heat transport qualitiesand will therefore be referred to as the auxiliary working fluid. Methylalcohol,

for example, has a freezing point l44F below that of water at the vaporpressures encountered in a heat pipe and would be suitable for use inthe inner section 14. Freon or ammonia would also be suitable.

To illustrate the operation of the heat pipe 10, it is assumed that theenvironment at the cold end of the heat pipe 10, shown generally at 22,is below the freezing point of the main working fluid in the outersection 12. It is further assumed that under high heat loads, the mainworking fluid is maintained liquid by the heat transferred to the coldend 22 from the warm end, shown generally at 24, where thermal inputenergy is applied. The transfer of heat occurs by heat pipe actioninvolving vaporization of the working fluid, transfer of the vapor fromthe warm end 24 to the cold end 22, condensation of vapor and transferof heat to the cold end 22, and return of the condensed fluid to thewarm end 24 by capillary action.

After a period of such operation, assume that the heat load or input atthe warm end 24 diminishes to a point where the cold end 22 of the heatpipe 10 drops in temperature below the solidification point of the mainworking fluid, and as a result of working fluid freezing at the cold end22, the wick 18 at the warm end 24 dries out. The lower freezing pointworking fluid in the inner section 14 is still in the liquid state. evenif the main working fluid solidifies. When an increase of heat flow intothe warm end 24 of the heat pipe 10 occurs, the heat will cause theauxiliary working fluid in the inner section 14 to vaporize andtransport its heat of vaporization to the cold areas of the innersection l4 at the cold end 22. The heat at the cold end of the innersection is transmitted through the wall of the inner section 14 to thewick 18 of the outer section directly in contact therewith and to thesolidified main working fluid, thereby melting the latter. As the mainworking fluid melts and returns to liquid form, it wets the wick 18 andredistributes itself throughout the wick 18. Now that the main workingfluid-has reestablished its liquid form throughout the wick 18, the mainouter section 12 of the heat pipe 10 regains control of heat pipeaction.

In a modified version of the heat pipe system above described, advantageis taken of the selective wettability of certain capillary materials orwicks towards different fluids. Capillary action is, of course,dependent on the wettability of the material ofthe wick by the specificworking fluid. The modification described below is based on therealization that many materials are wetted by one liquid but not byothers. For example, metallic copper is easily wetted by methyl alcoholbut not by water, whereas oxidized copper is easily wetted by bothmethyl alcohol and water.

Referring now to FIGS. 3 and 4, a main and an auxiliary heat pipe sharea common outer closed envelope 30 closed at both ends by plates 32. Theinterior surfaces of the envelope 30 and plates 32 are lined withalternating strips of two different wick materials 34 and 36 selectedfor their different wettability properties. The main wick material 34 isconstituted by wider strips on the cylindrical surface of the envelope30. The auxiliary wick material 36 is constituted by narrower strips onthe cylindrical surface of the envelope 30. The end plates 32 may becovered with alternate wide and narrow wedges of the main and auxiliarywick material 34 and 36, respectively. Alternatively, the end plates 32may be covered with the main wick material 34. The main wick material 34may be oxidized copper mesh, whereas the auxiliary wick material 36 maybe copper mesh.

Two different working fluids are provided within the envelope 30. One ofthe working fluids, such as water, constitutes the main working fluidand readily wets the oxidized copper or main wick material 34 but isrepelled by the copper or auxiliary wick material 36. The other workingfluid, such as methyl alcohol, constitutes the auxiliary working fluidand readily wets the copper or auxiliary wick material 36.

The combination of main working fluid and main wick material 34 withinthe envelope 30 can be considered as constituting a main heat pipe,whereas the combination. of auxiliary working fluid and auxiliary wickmaterial 36 within the same envelope 30 can be consid ered asconstituting an auxiliary heat pipe.

During normal operation under high heat loads, both working fluids willbe in liquid state and function as heat transport media. In condensingfrom vapor form,

the liquid water droplets of the main working fluid are repelled by theauxiliary wick material 36 and will soak only the main wick material 34.The condensing methyl alcohol vapors will soak the auxiliary wickmaterial 36 and may mix in part with the water in the main wick material34.

In a low temperature environment under reduced heat load conditions, thewater will freeze in the main wick material 34, leaving the auxiliarywick material 36 soaked with liquid methyl alcohol. The auxiliary heatpipe will continue to function after the main heat pipe has stoppedfunctioning. Thus, when the heat load increases again, the auxiliaryfluid will transport heat to the cold areas, thawing out the frozenwater until it all returns to liquid form again. When the water isentirely liquid again, it restores heat pipe action in the main heatpipe which now functions with greater efficiency than the auxiliary heatpipe.

The principal advantage of the invention is that it will permit theapplication of heat pipe-type devices in situations where, as a resultof varying heat flows or of variations in the external environment atthe cold or heat rejection areas of the heat pipe, freeze-up occurs in aworking fluid which has a high efficiency for heat transport. In such adevice, the large heat loads will be transferred by the main workingfluid which has been selected for maximum heat transport capability.

When the heat input into the device is reduced and less heat istransferred to the cold areas, the cold areas will drop below thesolidification temperature of the main working fluid. Circulation of themain working fluid will cease for the reasons described above. Theauxiliary working fluid will then carry the reduced heat transfer loads.Should the heat load now be increased, the auxiliary fluid will providethe needed heat transfer to the cold end of the device to start thawingor melting of the solidified main working fluid and start the processgoing again. The range of applications of heat pipes and heat pipe-typedevices is thereby vastly in creased.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. A heat pipe structure, comprising:

a. means forming a first heat pipe wall area of given longitudinalextent;

b. means forming a second heat pipe wall area joining said first heatpipe wall area substantially along their entire longitudinal extent;

c. first capillary means covering said first heat pipe wall area;

d. second capillary means covering said second heat pipe wall area;

e. said first and second capillary means being in thermal exchangerelationship with each other and with said first and second heat pipewall areas along their entire longitudinal extent;

f. a first working fluid arranged for transport through said firstcapillary means but not through said second capillary means;

g. a second working fluid arranged for transport through said secondcapillary means;

h. said first working fluid having a freezing point above the lowestexpected operating temperature of said heat pipe structure; and

i. said second working fluid having a freezing point below the lowestexpected operating temperature of said heat pipe structure whereby,during periods when said heat pipe structure is subjected to said lowestoperating temperature to cause solidification of said first workingfluid with consequent interruption therein of thermal transport action,said second working fluid will remain sufficiently fluid to continuefunctioning as a thermal transport medium.

2. The invention according to claim 1, wherein said first heat pipe wallarea forms a first enclosure means, and said second heat pipe wall areaforms a second enclosure means.

3. The invention according to claim 2, and further including meanssupporting said second enclosure means eccentrically within said firstenclosure means.

4. The invention according to claim 2, wherein said first enclosuremeans comprises a first tube, the interior surface of which is coveredwith said first capillary means;

said second enclosure means comprises a second tube, the interiorsurface of which is covered with said second capillary means;

and means joining the exterior surface of said second tube in thermalcontact with said first capillary means along contiguous arcuateportions thereof.

5. The invention according to claim 4, wherein said joining meanscomprises longitudinally spaced support members spring pressed betweensaid first and second tubes along surface portions thereof that areopposite said contiguous arcuate portions where said second tube andsaid first capillary means are held in thermal contact thereby.

6. The invention according to claim 4, wherein said second working fluidhas a freezing point substantially below that of water.

7. The invention according to claim 1, wherein said first and secondheat pipe wall areas join to form a single enclosure means, and saidfirst and second capillary means comprise sets of alternating stripscovering the interior surface of said enclosure means.

8. The invention according to claim 7, wherein one set of saidalternating strips is fabricated of a material that is readily wetted bysaid second working fluid but that repels said first working fluid;

and wherein the other set of said alternating strips is fabricated of amaterial that is readily wetted by said first working fluid.

9. The invention according to claim 8, wherein one set of saidalternating strips is fabricated of copper and the other set isfabricated of oxidized copper.

10. The invention according to claim 9, wherein said first working fluidconsists of water and said second working fluid consists of methylalcohol.

. UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 l777 811 Dated December 11 1973 Inventor(5) Arnold P Shlosinger It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

On the cover page, change the inventor's name from "Shcosinger" to--Shlosinger-- in both occurrences.

Insert the following within the first paragraph of the specification:

-The invention described herein was made in the performance of workunder a NASA contract and is subject to the provisions of Section 305 ofthe National Aeronautics and Space Act of 1958, Public Law 85-568 (72Stat. 435} 42 U.S.C. 2457) i Signed and sealed this 23rd day of April1371+.

I (SEAL) Attest:

QUE- ALL ll.l LL';TGiIJiH,JIi. G. MARSHALL UANN Attestir-Lg OfficerComissioner of Patents FORM PC4050 USCOMM-DC 60376-P69 {I U.$.GOVERNMENY FfllNTlNG OFFICE: 1,, 0-356-33

1. A heat pipe structure, comprising: a. means forming a first heat pipewall area of given longitudinal extent; b. means forming a second heatpipe wall area joining said first heat pipe wall area substantiallyalong their entire longitudinal extent; c. first capillary meanscovering said first heat pipe wall area; d. second capillary meanscovering said second heat pipe wall area; e. said first and secondcapillary means being in thermal exchange relationship with each otherand with said first and second heat pipe wall areas along their entirelongitudinal extent; f. a first working fluid arranged for transportthrough said first capillary means but not through said second capillarymeans; g. a second working fluid arranged for transport through saidsecond capillary means; h. said first working fluid having a freezingpoint above the lowest expected operating temperature of said heat pipestructure; and i. said second working fluid having a freezing pointbelow the lowest expected operating temperature of said heat pipestructure whereby, during periods when said heat pipe structure issubjected to said lowest operating temperature to cause solidificationof said first working fluid with consequent interruption therein ofthermal transport action, said second working fluid will remainsufficiently fluid to continue functioning as a thermal transportmedium.
 2. The invention according to claim 1, wherein said first heatpipe wall area forms a first enclosure means, and said second heat pipewall area forms a second enclosure means.
 3. The invention according toclaim 2, and further including means supporting said second enclosuremeans eccentrically within said first enclosure means.
 4. The inventionaccording to claim 2, wherein said first enclosure means comprises afirst tube, the interior surface of which is covered with said firstcapillary means; said second enclosure means comprises a second tube,the interior surface of which is covered with said second capillarymeans; and means joining the exterior surface of said second tube inthermal contact with said first capillary means along contiguous arcuateportions thereof.
 5. The invention according to claim 4, wherein saidjoining means comprises longitudinally spaced support members springpressed between said first and second tubes along surface portionsthereof that are opposite said contiguous arcuate portions where saidsecond tube and said first capillary means are held in thermal contactthereby.
 6. The invention according to claim 4, wherein said secondworking fluid has a freezing point substantially below that of water. 7.The invention according to claim 1, wherein said first and second heatpipe wall areas join to form a single enclosure means, and said firstand second capillary means comprise sets of alternating strips coveringthe interior surface of said enclosure means.
 8. The invention accordingto claim 7, wherein one set of said alternating strips is fabricated ofa material that is readily wetted by said second working fluid but thatrepels said first working fluid; and wherein the other set of saidalternating strips is fabricated of a material that is readily wetted bysaid first working fluid.
 9. The invention according to claim 8, whereinone set of said alternating strips is fabricated of copper and the otherset is fabricated of oxidized copper.
 10. The invention according toclaim 9, wherein said first working fluid consists of water and saidsecond working fluid consists of methyl alcohol.